We investigated phenomena of oxygen precipitation in silicon single crystals by two kinds of thermal treatment, supposing a CMOS fabrication process. The one consisted of the first annealing at 1123K for 4 hrs and the second annealing at 1423K for 16 hrs. The other one consisted of the annealing at 1273K for 4 hrs and the second annealing at 1423K for 13 hrs. In the results, a single-step preannealing at 723K for 2 hrs was effective for the oxygen precipitation by the former process and nonuniform distribution profiles along crystal growth axis were well improved, however, insufficiently improved against the latter process. We considered a two-step preannealing process consisting of the first annealing at 723K for 2 hrs and the second annealing at 923K for 2 hrs. This new process was effective for the oxygen precipitation by the latter process. Especially, we could obtain uniform distribution profiles of oxygen precipitation along a crystal growth axis.